Is there a way to update Blender without deleting it and downloading the new version?
Blender doesn't have an (auto-)update function that would do this for you.

Some years ago I experimented with a way to create binary patches - that works pretty ok, but the experiment never took further form, as an entire infrastructure to support this needs to be created, and for such an effort no resources have been found (people willing to work on it, maintain it and the servers).

Until that day you'll just have to download the entire program. Note that you don't have to install Blender, you can just run it from the location you downloaded and extracted it to (at least on Linux and Windows, so assuming it works also on OS X).